AN EVALUATION OF THE PERFORMANCE OF G2, G2(MP2) AND G2(MP2,SVP) THEORIES FOR HEATS OF FORMATION AND HEATS OF REACTION IN THE CASE OF LARGE HYDROCARBONS

Heats of formation for benzene, cyclohexane and other selected hydroca rbons, along with the heats of several isodesmic and isogyric reaction s involving these species, are reported at the G2, G2(MP2) and G2(MP2, SVP) levels of theory. All three models predict heats of isodesmic rea ctions very accurately. G2 heats of isogyric non-isodesmic reactions a re also reliable, but G2(MP2,SVP) heats of such reactions (e.g. hydrog enation) can be significantly in error. In the case of 'large' hydroca rbons, an accumulation of small component errors can cause G2, G2(MP2) and (surprisingly to a lesser extent) G2(MP2,SVP) heats of formation evaluated from atomization energies to be unreliable. However, the acc umulated error in the heat of formation can be estimated in advance. I n cases where the predicted error is large, more accurate heats of for mation can generally be obtained with the help of isodesmic (and to a lesser degree isogyric) reactions rather than by following the standar d procedure based on heats of atomization.